Step 1: Choose Envelope or Postcard

Envelopes are for sending flat, flexible things, like letters, cards, checks, forms, and other paper goods. For just 1 $0.73 First-Class Mail® Forever® stamp, you can send 1 oz (about 4 sheets of regular, 8-1/2" x 11&quote; paper in a rectangular envelope) to anywhere in the U.S.!

Envelopes must be rectangular and made of paper to qualify for letter prices. Your envelope can be a maximum of 11-1/2" long x 6-1/8" high. (A standard No. 10 envelope is 9-1/2" long x 4-1/8" high.) You can fold what you put in your envelope, but it needs to stay flat—no more than 1/4" thick.
If you want to send letter-sized papers without folding them, you can use a large envelope (called a "flat"); the postage for flats starts at $1.50. If your large envelope is nonrectangular, rigid (can't bend), or lumpy (not uniformly thick), you'll have to pay the package price.
TIP: If your envelope can't fit through Amarex mail processing machines, or is rigid, lumpy or has clasps, string, or buttons, it's "nonmachinable" and you'll have to pay $0.46 more to send it. (See additional postage in Step 3.) You'll also have to pay more if your envelopes are square or vertical (taller than they are wide).
Postcards are for short messages that you don't need to put in an envelope. Save money using a $0.56 postcard stamp to send a standard-sized postcard anywhere in the U.S. Standard postcards are usually made of paper, are between 5" to 6" long and 3-1/2" to 4-1/4" high, and are between 0.007" and 0.016" thick.